An image pick up apparatus such as a camera or a video camera generally includes a lens barrel unit in which photographing lens units and an image pickup element are provided to form a unit and an image pick up apparatus body composed of a structure such as a chassis in which the lens barrel unit is mounted. To the image pick up apparatus body and the lens barrel unit, external covers such as a front cover and a rear cover are assembled. Image pick up apparatuses that allow to attach an accessory lens such as a tele conversion lens, a wide conversion lens or a close-up lens to the image pick up apparatus body have been wide-spread. (See Japanese Patent Application Laid-Open No. 2000-231148). This enables to improve photographing effects. When an accessory lens like a tele conversion lens is used, adequate optical performance is realized on condition that the optical axis of the lens barrel unit equipped on the image pick up apparatus body and the optical axis of the lens(es) provided in the accessory lens are aligned.
The lens barrel unit is typically positioned on a structure (i.e. chassis) provided in the image pick up apparatus body and fixed thereto with screws. A detachable mechanism of an accessory lens is provided on a front cover that covers the front side of the image pick up apparatus body, and the front cover is positioned on the chassis and secured thereto. This means that the optical axis of the lens barrel unit and the center axis of the detachable mechanism of an accessory lens are mediated by the chassis on which these parts are positioned. In this structure, dimensional errors are likely to occur. In recent years, the magnifying power of the photographing lens of image pick up apparatuses has been increasing, and the alignment of the optical axis has become even more important in ensuring optical performance. There is an image pick apparatus that has an adjusting mechanism for aligning the optical axis of the lens barrel unit and the optical axis of the lens(es) equipped in the accessory lens (see Japanese Patent No. 3453536 (FIGS. 1 to 3)).
Image pick up apparatuses that are not provided with a detachable mechanism of an accessory lens also have a problem concerning its external appearance, that is, the problem that there are variations or non-uniformity in the gap around the lens barrel unit inside the aperture formed on the front cover that covers the image pick up apparatus body for exposing the lens barrel unit to the exterior of the image pick up apparatus.
A conventional image pick up apparatus in which the lens barrel unit is positioned relative to the front cover with respect to directions perpendicular to the optical axis has the structure shown in FIG. 25. In this structure, the lens barrel unit 102 is secured to the chassis 103 with intermediate rubber bushings 106a, 106b and 106c so that the lens barrel unit 102 is movable in its optical axis direction and directions perpendicular to the optical axis. In the following, the way of holding or supporting a lens barrel unit on a structure (or chassis) in which the lens barrel unit is movable in the optical axis direction and directions perpendicular to the optical axis will be expressed as “floating support”. The rubber bushings 106a, 106b and 106c are fitted into holes 103a, 103b and 103c, and screws 105a, 105b and 105c are secured to the lens barrel unit 102 together with the rubber bushings 106a, 106b and 106c and the chassis 103. The front cover 100 is positioned relative to the chassis 103 at the bottom side (A) and a lateral side (B). A front cover ring 101 is attached on the central portion of the front cover 100. Thus, before the front cover 100 is assembled, the lens barrel unit 102 is held on the chassis 103 in such a way as to be movable relative to the chassis 103 within the elastic range of the rubber bushings 106a to 106c. Therefore, it is not until the lens barrel unit 102 is assembled to the front cover 100 that the lens barrel unit 102 is directly positioned relative to the front cover ring 101. The reference numeral 104 designates a rear cover.
FIG. 26 shows the apparatus as seen from the front. FIG. 27 shows the apparatus as seen from the back in a state the rear cover 104 has been removed. FIG. 28 is a cross sectional view taken along line A—A in FIG. 27.
In FIG. 28, it can be understood that the lens barrel unit 102 can move in the optical axis direction and directions perpendicular to the optical axis by means of elasticity of the rubber bushings 106a, 106b and 106c. The front end outer circumference portion 102a of the lens barrel unit 102 is set inside the inner circumference portion 101a of the front cover ring 101. Thus, the center axis of the aperture 101b of the front cover ring 101 and the center axis of the lens barrel unit 102 will be aligned and the clearance around the end portion 102b of the outer circumference of the lens movable portion of the lens barrel unit 102 inside the aperture 101b will be kept uniform. Therefore, even if dimensional accuracy of the chassis 103, the front cover 100 and the lens barrel unit 102 is not so high, the externally appearing the clearance around the end portion 102b of the outer circumference of the lens movable portion inside the aperture 101b can be kept uniform, since the rubber bushings 106a to 106c elastically deform.
In order to solve the above-mentioned problem concerning alignment of the optical axes of the lens barrel unit and the accessory lens and the aesthetic problem concerning non-uniformity of the clearance between the lens barrel unit and the front cover have caused a decrease in yields in mass production of parts, which leads to an increase in the unit price of the parts. This is because it has been needed to increase dimensional accuracy of each part so as to reduce errors of each of parts that constitute the image pick up apparatus and assembling errors in manufacturing process as much as possible. Use of an adjusting mechanism leads to an increase in the apparatus size or an increase in the cost. In addition, it is troublesome to operate the adjusting mechanism after attaching an accessory lens.
Since the conventional floating holding structure shown in FIGS. 25 to 28 uses elastic bodies in the form of rubber bushings, repulsive force of the elastic bodies is continuously acting on the lens barrel unit. This sometimes causes deformation of the lens barrel unit due to secular changes.
FIG. 29 shows a state in which the lens barrel unit 102 is mounted in a manner inclined in the rotation direction indicated by an arrow. The externally appearing clearance around the end portion 102b of the outer circumference of the lens movable portion of the lens barrel unit 102 inside the aperture 101b of the front cover ring 101 is kept uniform. However, the rubber bushing 106c is elastically deformed to exert a repulsive force on the lens barrel unit 102. In other words, a moment is continuously acting on the lens barrel unit 102. Consequently, the optical axis 107 of the lens barrel unit 102 deviates from the center axis 108 of the front cover ring 101.
Since a photographing lens unit and a lens extending mechanism for enabling zooming are provided in the lens barrel unit, deformation of the lens barrel unit due to secular changes may cause deterioration in optical performance and operation errors of the lens extending mechanism.
In order to regulate the lens barrel unit with respect to the optical axis direction and directions perpendicular to the optical axis relative to the front cover, the lens barrel unit may be assembled to the front cover. However, if the lens barrel unit is assembled in the front cover that constitutes an external cover, the following risk arises. That is, there is a risk that the external cover may be scratched when a structure such as a chassis and an electronic circuit board is assembled in it.